Add avatar icon as login property w/ default.

[mjarosch]

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This is a pull request for issue #9916. It adds an avatarIcon property to the Login component w/ a default of the Lock icon. This allows for overriding the icon without having to copy the complete component.

Ideally, this would be included as part of a v4 point release. This is my first pull request, so please let me know if there is anything else needed.

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Fix https://github.com/marmelab/react-admin/issues/9916